Urine tests

Examine mid-stream urine (MSU) whenever you suspect renal disease.


Dipstick:

• Haematuria:
• Renal causes:
• Neoplasia,
• glomerulonephritis (often IgA nephropathy, p288),
• tubulointerstitial nephritis,
• polycystic kidney,
• papillary necrosis,
• infection (pyelonephritis),
• trauma.
• Extrarenal:
• Calculi,
• infection (cystitis, prostatitis, urethritis),
• neoplasia (bladder, prostate, urethra),
• trauma (eg from catheter).
• Tests:
• Urine MC&S, FBC, ESR, CRP, U&E, clotting.
• Others:
• AXR/KUB, p284 (stones), urine cytology, estimation of proteinuria (see below), renal ultrasound ± renal biopsy.  
• Management:
• Usually refer first to a urologist, and do ultrasound.
• Only refer initially to renal physician if the risk of urothelial malignancy is low and risk of glomerulonephritis is not negligible (eg <40yrs old, creatinine↑, BP↑, proteinuria, systemic
  symptoms, family history of renal disease).
• Not all women with recurrent UTI + haematuria need cystoscopy, but have a good reason not to do cystoscopy (Reynard's rule).
• False +ve dipstick haematuria:
• Haemoglobinuria,
• myoglobin (eg in rhabdomyolysis),
• beetroot,
• porphyria,
• alkaptonuria,
• rifampicin,
• phenindione,
• phenolphthalein.
• Proteinuria:
• Normal protein excretion is <150mg/d, consisting of <30g/d of albumin.
• Causes
• Renal causes of proteinuria:
• UTI,
• orthostatic proteinuria,
• glomerulonephritis (GN),
• ↑BP,
• DM,
• myeloma,
• amyloid.
• Extrarenal:
• Fever,
• exercise,
• pregnancy,
• CCF,
• vaginal mucus,
• recent ejaculation.
• Tests:
• BP, urine MC&S.
• Estimation of proteinuria:
• 24h urine collection for protein and creatinine quantifies proteinuria if collected accurately:
• spot tests for urine albumin:creatinine ratio or urine protein:creatinine index are much easier and provide reasonably accurate information;
• renal ultrasound;
• autoantibodies eg immunoglobulins, serum electrophoresis, urinary Bence Jones protein (p288);
• consider a renal biopsy if renal function is deteriorating.
• Microalbuminuria is undetectable on dipstick, with albuminuria of 30-300mg/24h on lab tests.
• Causes: DM, ↑BP, minimal change GN.
• Other substances—
• Glucose:
• Low renal threshold (eg chronic renal failure), DM, pregnancy, sepsis, renal tubular damage.
• Ketones: Starvation, ketoacidosis. 
• Leucocytes: UTI, vaginal discharge.
• Nitrites: UTI, high-protein meal.
• Bilirubin: Obstructive jaundice.
• Urobilinogen: Pre-hepatic jaundice.
• Specific gravity: Normal range: 1.000-1.030 (useful to assess degree of proteinuria or haematuria).
• pH: Normal range: 4.5-8 (acid-base balance: p658).

Microscopy

• Put a drop of fresh urine (MSU or suprapubic aspirate) on a microscope slide, cover with a coverslip and examine under low (×100) and high (×400) power for leucocytes, red cells, bacteria, casts and
  crystals.
• If renal disease is suspected, a centrifuged urine should be examined.
• Leucocytes:
• >10/mm3 in an unspun urine specimen is abnormal. 
• Usually due to a UTI, see p283 for causes of sterile pyuria (when no bacteria are found).
• Causes of sterile pyuria
• Always remember renal TB (do 3 early morning urines).
• Treated UTI <2 weeks prior
• Inadequately treated UTI
• Appendicitis
• Calculi
• Prostatitis
• Bladder tumour
• UTI with fastidious culture requirement
• Papillary necrosis (eg DM or analgesic excess)
• Tubulointerstitial nephritis
• Polycystic kidney
• Chemical cystitis (eg cyclophosphamide).
• Red cells:
• >2/mm3 in unspun urine is abnormal.
• Causes: See haematuria.
• Consider their morphology to understand where in the GU tract they come from.
• If >10% of RBCs are dysmorphic G1 cells, suspect glomerular bleeding, and look hard for red cell casts.
• G1 cells have doughnut shapes, target configurations, and membrane
  protrusions or blebs.
• NB: identifying dysmorphic red cells is subjective and often difficult.
• Acanthocyturia ≈ RBCs with spicules.
• G1 cell images (stained urine cytology): www.uninet.edu/cin2003/conf/nguyen/nguyen.html 

Casts

• are cylindrical bodies formed in the lumen of distal tubules.
• Finely granular and hyaline casts (clear, colourless) are found in normal concentrated urine.
• They are increased in fever, exercise or loop diuretics.
• Densely granular:
• Glomerular or tubular disease eg GN, interstitial nephritis.
• Fatty casts:
• Moderate-heavy proteinuria.
• Don't mistake fat globules for RBCs.
• Red cell casts are a diagnostic marvel, as they prove that haematuria is glomerular, allowing you to start an interesting dialogue with a nephrologist: ‘is there vasculitis (p542), glomerulonephritis,
  or malignant hypertension?’
• White cell casts occur in pyelonephritis.
• Tubular cell casts occur in acute tubular necrosis.

Crystals

• are common in old or cold urine and may not signify pathology.
• They are important in stone formers:
• cystine crystals are diagnostic of cystinuria, and
• oxalate crystals in fresh urine may indicate a predisposition to form calculi.

24h urine

• for Na+, K+, Ca2+, urea, creatinine ± protein excretion.
• Take blood simultaneously for creatinine to calculate creatinine clearance (p661).